formulary_spec: update API tests to avoid mocking

Stop mocking the formulary loader method. We just need to set the
environment variable so that it knows to load things from the API.

Fix spec that doesn't work with the `CoreTap.formula_names`.
That method assumes that we always have a valid `ruby_source_path`
in the API JSON even though that was optional for a while after
launching the API if my memory serves me. It's probably fine
to assume this should always be set though and I changed it to use
`Hash#fetch` to give use better error messages if something goes wrong in the future.

Remove unused `allow(x).to receive(y).and_call_original` in tap spec.

Library/Homebrew/tap.rb

@@ -1169,7 +1169,8 @@ def formula_files_by_name
       name, formula_hash = item
       # If there's more than one item with the same path: use the longer one to prioritise more specific results.
       existing_path = hash[name]
-      new_path = File.join(tap_path, formula_hash["ruby_source_path"]) # Pathname equivalent is slow in a tight loop
+      # Pathname equivalent is slow in a tight loop
+      new_path = File.join(tap_path, formula_hash.fetch("ruby_source_path"))
       hash[name] = Pathname(new_path) if existing_path.nil? || existing_path.to_s.length < new_path.length
     end
   end

Library/Homebrew/test/formulary_spec.rb

@@ -158,8 +158,6 @@ class Wrong#{described_class.class_s(formula_name)} < Formula
 
     context "with installed Formula" do
       before do
-        allow(described_class).to receive(:loader_for).and_call_original
-
         # don't try to load/fetch gcc/glibc
         allow(DevelopmentTools).to receive_messages(needs_libc_formula?: false, needs_compiler_formula?: false)
       end
@@ -329,6 +327,7 @@ def formula_json_contents(extra_items = {})
               "run_type"    => "immediate",
               "working_dir" => "/$HOME",
             },
+            "ruby_source_path"         => "Formula/#{formula_name}.rb",
           }.merge(extra_items),
         }
       end
@@ -378,7 +377,7 @@ def formula_json_contents(extra_items = {})
       end
 
       before do
-        allow(described_class).to receive(:loader_for).and_return(described_class::FormulaAPILoader.new(formula_name))
+        ENV.delete("HOMEBREW_NO_INSTALL_FROM_API")
 
         # don't try to load/fetch gcc/glibc
         allow(DevelopmentTools).to receive_messages(needs_libc_formula?: false, needs_compiler_formula?: false)